OBJECTIVES: 1. To characterize the size and cell cycle properties of various erythroid progenitor compartments in normal man and in patients with selected disease states associated with anemia or polycythemia. 2. To determine the cellular interactions capable of enhancing in vitro erythroid growth through the fractionation and identification of subpopulations of human lymphocytes. To study possible mechanisms of cell mediated suppression of marrow growth in selected human disease states. 3. To employ an in vitro assay for erythropoietin to the differential diagnosis of polycythemic states in man and to studies of the kinetics of erythropoietin turnover in selected animal models.